Vicinity Of Fracture Research Articles

Comparison of Supercritical CO2 With Water as Geofluid in Geothermal Reservoirs With Numerical Investigation Using Fully Coupled Thermo-Hydro-Geomechanical Model

Abstract In the present work, fully coupled dynamic thermo-hydro-mechanical (THM) model was employed to investigate the advantage and disadvantages of supercritical CO2 (SCCO2) over water as geofluids. Low-temperature zone was found in both SCCO2-enhanced geothermal system (EGS) and water-EGS systems, but spatial expansion is higher in water-EGS. Although, the spatial expansion of SCCO2 into the rock matrix will help in the geo-sequestration, the expansion of stress and strain invaded zones were identified significantly in the vicinity of fracture and injection well. SCCO2-EGS system is giving better thermal breakthrough and geothermal life conditions compared to the water-EGS system. Reservoir flow impedance (RFI) and heat power are examined, and heat power is high in the water-EGS system. Minimum RFI is found in the SCCO2-EGS system at 45 °C and 0.05 m/s. Maximum heat power for SCCO2-EGS was observed at 35 °C, 20 MPa, and 0.15 m/s. Therefore, the developed dynamic THM model is having greater ability to examine the behavior of SCCO2-EGS and water-EGS systems effectively. The variations occur in the rock matrix, and the performance indicators are dependent on the type of fluid, injection/production velocities, initial reservoir pressure, and injection temperature. The advantages of SCCO2-EGS system over the water-EGS system provide a promising result to the geothermal industry as a geofluid.

Correlation of Acoustic Emissions with Electrical Signals in the Vicinity of Fracture in Cement Mortars Subjected to Uniaxial Compressive Loading

Acoustic emissions (AEs) and weak electrical signals, also known as pressure stimulated currents (PSCs), were concurrently recorded in order to investigate their behavior and detect precursory indicators when cement mortar specimens were subjected to mechanical compressive loading, emphasizing the behavior of the AEs and the PSC signal in the vicinity of fracture. The axial compressive loading protocol incorporated a constantly increasing stress, from early stress values up to the vicinity of fracture and a sequential stress stabilization until the time the specimen collapses, due to severe growing internal damages. Concurrent recordings of the electrical and acoustic emissions were performed. The AE recordings were analyzed, by incorporating the recently introduced F- and P-functions, and the well-known b-value. The experimental results highlight strong similarities regarding the variations of the PSC signal, the AE hits occurrence rate (F-function), and the AE hits energy release rate (P-function). The above was also confirmed with another similar experiment in an identical specimen. It is noteworthy that, during the stay of the specimens under a constant load regime near their strength levels, a peak appears in the above quantities, which is directly related to an increased rate of axial deformation. The temporal evolution of the b-values is also presented. Results show that the local minima appearing at values close to b ≈ 1.0 correspond to the local maxima of the PSC signal. It is straightforwardly concluded that when both the PSC signal and the AE data are combined, they provide clear pre-failure indicators.

Adaptive conservative time integration for transport in fractured porous media

The time step of an explicit time integration scheme for solving time-dependent hyperbolic partial differential equations in a finite volume method (FVM) framework is restricted by the Courant–Friedrichs–Lewy (CFL) criterion. Conventional time stepping integrates all grid cells with the same time step. This causes unnecessary computational costs when wave speeds and/or grid spacing vary considerably throughout the domain, and a few critical cells dictate the global time step, although most cells could be advanced with a much larger time step. Adaptive time stepping overcomes this issue by allowing different local time step sizes for each grid cell. The adaptive conservative time integration (ACTI) scheme is a recently developed adaptive time stepping method which relies on local time steps that are equal to the largest time step divided by powers of two.This work extends the ACTI scheme to tracer transport in fractured porous media. When fluid velocity within highly permeable fractures is higher than in the rock matrix and local grid refinement is applied around fractures, the CFL restriction would require prohibitively small time steps in the vicinity of fractures. For two-dimensional discrete fracture and matrix models of fracture patterns we demonstrate that ACTI reduces the computational cost by orders of magnitude compared to global time stepping while retaining solution accuracy. Empirically, we show that ACTI is stable in combination with a first-order explicit flux discretization scheme. Since combination with a standard higher-order MUSCL scheme can lead to spurious oscillations in the solution, we propose a modified MUSCL scheme relying on advection of an inclined reconstruction (MUSCL-AIR).

Experimental Study on the Thermal Infrared Spectral Variation of Fractured Rock

Previous studies have shown that thermal infrared radiation (TIR) anomalies occur in the vicinity of fractures that form when a rock is loaded to failure. Different types of fracturing modes correspond to different TIR anomaly trends. However, the spectral features and the mechanisms responsible for the TIR changes in the fracturing stage remain poorly understood. In this paper, experiments involving observations of the thermal infrared spectrum (8.0–13.0 μm) of loaded sandstone during the fracturing stage were conducted under outdoor conditions. The experiment yielded the following results: (1) Different fracturing modes can lead to different trends in the spectral radiance variation; (2) when an extensional fissure appeared on the rock surface, the radiance increased with a local peak in the 8.0–9.7 μm range; (3) when local bulging formed at the surface, the radiance decreased, with a local valley in the 8.0–9.7 μm range. The radiance variation caused by morphologic changes is the combined result of changes in both the temperature and the emissivity. The characteristic waveband corresponding to the reststrahlen features (RF) of quartz was mainly related to the emissivity change. This study provides a preliminary experimental foundation for the detection of crustal surface fractures via satellite-based remote sensing technology.

Comparison of Supercritical CO <sub>2</sub> with Water as Geofluid in Geothermal Reservoirs with Numerical Investigation Using Fully Coupled Thermo-Hydro-Geomechanics

In the present work, fully coupled dynamic thermo-hydro-geo-mechanical model was employed to investigate the advantage and disadvantages of supercritical CO2(SCCO2) over water as geofluids. Low-temperature zone was found in both SCCO2-EGS and water-EGS systems, but spatial expansion is higher in water-EGS. Although, the spatial expansion of SCCO2 into the rock matrix will help in the geo-sequestration. The expansion of stress and strain invaded zones were identified significantly in the vicinity of fracture and injection well. SCCO2-EGS system is giving better thermal breakthrough and geothermal life conditions compared to the water-EGS system. Reservoir flow impedance (RFI) and heat power are examined, and heat power are high in the water-EGS system. Minimum RFI is found in the SCCO2-EGS system at and 0.05 m/s. Maximum heat power for SCCO2-EGS was observed at , 20 MPa, and 0.15 m/s. Therefore, the developed dynamic THM model is having greater abilities to examine behavior of SCCO2-EGS and water-EGS systems effectively. The variations occur in the rock matrix and the performance indicators are dependent on the type of fluid, injection/production velocities, initial reservoir pressure, injection temperature. The advantages of SCCO2-EGS system over the water-EGS system, providing a promising result to the geothermal industry as geofluid.

Novel Approaches for Fracture Detection in Steel Girder Bridges

The bottom flanges of steel plate girder bridges can be considered fracture-critical elements depending on the number of girders and bridge configuration. For such cases, it is required that inspection of these bridges be carried out using costly “arms-length” approach. New techniques in structural health monitoring (SHM) that use non-contact sensors and self-powered wireless sensors present alternative approach for inspection. Application of such techniques would allow timely detection and application of repair and strengthening, in other word, providing for more resilient bridges. This paper investigates the feasibility of using a handful of self-powered wireless or non-contact sensors for continuous or periodic monitoring and detection of fracture in steel plate girder bridges. To validate this concept, vibration measurements were performed on an actual bridge in the field, and detailed finite element analyses were carried out on a multi-girder bridge. The records obtained show that vibration amplitude was significantly increased for fractured girder, and a distinct pattern of strain variation was registered in the vicinity of fracture, all of which can be detected effectively with relevant sensors. Moreover, the amplitude and frequency of the vibration was shown to be significant enough for providing the required power for typical sensor(s).

A semi-analytical model for characterizing transient flow behavior of reoriented refractures

Abstract In order to improve the field productivity, the industries can create new fractured wells to reduce the interval between the initial fractures (initial fracture indicates a fracture that is induced during the initial fracturing treatment). Production from the initial fractured wells can induce stress reorientation in the vicinity of the fractures. As such, a refracturing treatment in the stress reorientation region can lead to a reoriented refracture which has a certain azimuth with respect to the initial fractures. This azimuth can even be 90° if the stress reorientation is sufficiently large. In addition to the conventional parameters (including refracture's length, refracture's conductivity, and the in-situ conditions), there are two more factors, the reorientation azimuth and the interference from the initial fracture, which can exert significant influence on the transient flow behavior of the reoriented refractures. In such case, the conventional analytical/semi-analytical models neglecting these two factors are no longer applicable to characterize the transient flow behavior from such a refracture. In this work, we develop a novel semi-analytical model to characterize the transient flow behavior of a reoriented refracture considering the interference from the initial fractures. In this model, the fractures are explicitly represented with discretized segments. We apply finite difference approximation to the initial fractures and refractures, respectively, to simulate the transient flow in the fracture system. In addition, the fluid flow in the matrix system can be characterized by Green function and Newman product method. Based on the continuity of flux and pressure, we couple the fracture flow equations with the matrix flow equations to construct a semi-analytical model. With the aid of the proposed model, we conduct a thorough investigation of the transient flow behavior of reoriented refractures. It is observed that, at the early production period, two fracture pseudo-steady-state flows can appear due to the interference from the initial fractures. The formation linear flow and bilinear flow can also be observed during the production period of a refractured well. In an anisotropic-permeability reservoir, the refracture's azimuth and position can exert a significant impact on the productivity of the fractures. There is a region in the vicinity of the initial fractures, within which an orthogonal refracture can lead to the highest field productivity.

Granite microporosity changes due to fracturing and alteration: secondary mineral phases as proxies for porosity and permeability estimation

Abstract. Several alteration facies of fractured Lipnice granite are studied in detail on borehole samples by means of mercury intrusion porosimetry, polarized and fluorescent light microscopy, and microprobe chemical analyses. The goal is to describe the granite void space geometry in the vicinity of fractures with alteration halos and to link specific geometries with simply detectable parameters to facilitate quick estimation of porosity and permeability based on, for example, drill cuttings. The core of the study is the results of porosity and throat size distribution analyses on 21 specimens representing unique combinations of fracture-related structures within six different alteration facies basically differing in secondary phyllosilicate chemistry and porosity structure. Based on a simple model to calculate permeability from the measured porosities and throat size distributions, the difference in permeability between the fresh granite and the most fractured and altered granite is 5 orders of magnitude. Our observations suggest that the porosity, the size of connections and the proportion of crack porosity increase with fracture density, while precipitation of iron-rich infills as well as of fine-grained secondary phyllosilicates acts in the opposite way. Different styles and intensities of such end-member agents shape the final void space geometry and imply various combinations of storage, transport and retardation capacity for specific structures. This study also shows the possibility to use standard mercury intrusion porosimetry with advanced experimental settings and data treatment to distinguish important differences in void space geometry within a span of a few percent of porosity.

Grid-Sensitivity Analysis and Comparison Between Unstructured Perpendicular Bisector and Structured Tartan/Local-Grid-Refinement Grids for Hydraulically Fractured Horizontal Wells in Eagle Ford Formation With Complicated Natural Fractures

Summary In the context of modeling fractured horizontal wells, unstructured grids have been applied to generate simulation meshes for complex fracture networks. It is necessary to investigate how to choose an unstructured mesh to accurately simulate production performance. In this paper, a new unstructured gridding and discretization work flow is proposed to handle nonorthogonal and low-angle intersections of extensively clustered fractures with nonuniform apertures. The work flow is then validated with two models in terms of production behavior and central-processing-unit (CPU) performance: a synthetic model with one horizontal well and orthogonal intersected hydraulic fractures built by tartan grid, and a field-scale local-grid-refinement (LGR) model with three horizontal wells and irregular hydraulic fractures in a slightly dipping reservoir created by a commercial software plug-in. Good-quality matches are obtained between unstructured and structured grids in both pressure and production behavior. Sensitivity analysis of the meshing parameters suggests that refinement in the vicinity of fractures has improved both early and late production of a well, whereas background density has a dominant effect on the late production. Background-grid type and orientation have less influence as long as they have the same grid density. Fewer cells can be achieved by increasing reservoir-background size and size-progression ratio, replacing unstructured-background grids with structured grids, and reducing the complexity of the fracture networks without loss of the accuracy, resulting in improved CPU performance. This study applies unstructured grids to simulate multiple horizontal wells with complicated fracture networks, and provides detailed comparisons between unstructured and structured grids. Most importantly, it resolves the question regarding how to choose an appropriate mesh to yield both accurate results and high-quality CPU performance.

Seismic performance evaluation of Egyptian code-designed steel moment resisting frames

Despite the fact that steel is a ductile material, the significant damage, during earthquake events, highlighted the need to thoroughly investigate the seismic performance of steel structures. Seismic design procedures have been developed to enable structures to achieve specific acceptable level of damage under dynamic loads in accordance with particular levels of ductility. The ductility of steel moment resisting frames is developed through flexural yielding of beams, shear yielding of column panel zones, and flexural yielding of columns. Meanwhile, the frame must develop the required ductility without failure in the beam-to-column connection. The observations on panel zone behavior revealed that it can afford high ductility; however, localized deformations at corners of panel zone may increase the likelihood of fracture in vicinity of beam flange welds. On the other hand, the observations on flexural yielding behavior of columns reported potential soft story collapse. Consequently, counting on ductility due to shear yielding of panel zone and flexural yielding of columns is not recommended. Hence, the focus of this study has been directed toward flexural yielding of frame beams. The effect of beam profile slenderness (according to the Egyptian code design limits) has been examined against ductility, over-strength and redundancy with numerical assessment for the anticipated seismic force reduction factor. The reduction factor has been evaluated using both the N2-method and the time history analysis method. Accordingly, a guideline has been established for the Egyptian code provisions to enable professionals to assign the steel moment resisting frame between adequate-ductility, intermediate-ductility and limited-ductility.

Biomechanical assessment of composite versus metallic intramedullary nailing system in femoral shaft fractures: A finite element study

BackgroundIntramedullary nails are the primary choice for treating long bone fractures. However, complications following nail surgery including non-union, delayed union, and fracture of the bone or the implant still exist. Reducing nail stiffness while still maintaining sufficient stability seems to be the ideal solution to overcome the abovementioned complications. MethodsIn this study, a new hybrid concept for nails made of carbon fibers/flax/epoxy was developed in order to reduce stress shielding. The mechanical performance of this new implant in terms of fracture stability and load sharing was assessed using a comprehensive non-linear FE model. This model considers several mechanical factors in nine fracture configurations at immediately post-operative, and in the healed bone stages. ResultsPost-operative results showed that the hybrid composite nail increases the average normal force at the fracture site by 319.23N (P<0.05), and the mean stress in the vicinity of fracture by 2.11MPa (P<0.05) at 45% gait cycle, while only 0.33mm and 0.39mm (P<0.05) increases in the fracture opening and the fragments' shear movement were observed. The healed bone results revealed that implantation of the titanium nail caused 20.2% reduction in bone stiffness, while the composite nail lowered the stiffness by 11.8% as compared to an intact femur. InterpretationOur results suggest that the composite nail can provide a preferred mechanical environment for healing, particularly in transverse shaft fractures. This may help bioengineers better understand the biomechanics of fracture healing, and aid in the design of effective implants.

Optimization of Fracture Spacing in Western Sichuan Tight Gas Based on Fracture Interference

Western Sichuan Depression is a typical tight gas reservoir, the effective stimulation methods for such reservoir is staged fracturing in horizontal wells. But, scholars failed to give the method to set reasonable fracture spacing in tight gas. In this paper, Western Sichuan tight gas reservoir continuous sand body model is established by numerical simulation, through which the gas reservoir production performance in different facture spacing can be learned. Numerical pressure in the vicinity of fracture to describe the scale of fracture interference by the law of pressure drop super position. Then, net present value is used to optimize fracture spacing. The result show that 73 m is the optimal fracture spacing of II B continuous sand body in XinMa blcok .The research method is not only applicable to continuous sand body of tight gas reservoirs, but can be extended to non-continuous sand body. Furthermore it plays a constructive role in promoting the fracturing exploitation in tight gas.

Roles of coal heterogeneity on evolution of coal permeability under unconstrained boundary conditions

Coal permeability models based on constrained conditions such as constant volume theory can successfully match unconstrained experimental data and field observations. However, these models have a boundary mismatch because the boundary of permeability models is constrained while experiment boundary is free displacement or unconstrained. What the mechanism is to require such a boundary mismatch has not been well understood. In this study, a full coupled approach was developed to explicitly simulate the interactions of coal matrixes and fractures. In this model, a matrix-fracture model is numerically investigated after incorporating heterogeneous distributions of Young's modulus, Langmuir strain constant in the vicinity of the fracture. The impact of these local heterogeneities of coal mechanical and swelling properties on the permeability evolution is explored. The transient permeability evolution during gas swelling process is investigated and the difference between the final equilibrium permeability and transient permeability is compared. With the heterogeneity assumption, a net reduction of coal permeability is achieved from the initial no-swelling state to the final equilibrium state. This net reduction of coal permeability increases with the fracture (injection) pressure and is in good agreement with laboratorial data under the unconstrained swelling conditions. Coal local heterogeneity in vicinity of fracture can therefore be the mechanism of the above mismatch.

Melt– and Fluid–Rock Interaction in Supra-Subduction Lithospheric Mantle: Evidence from Andesite-hosted Veined Peridotite Xenoliths

We report petrographic, major and trace element data for xenoliths from the andesitic Avacha volcano (Kamchatka), which host orthopyroxene (opx)-rich veins of mantle origin formed either by rapid crystallization of intruded melts or by their interaction with the host harzburgite. Studies of such veins may give better insights into sub-arc mantle processes (in particular on a millimetre to centimetre scale) than those of (1) arc xenoliths that do not preserve solidified initial metasomatizing agents, (2) massif peridotites, probably modified during their emplacement, or (3) arc magmatic rocks, which provide indirect information. We seek to trace the evolution of these agents as they react with the host peridotite and to assess their impact on the wall-rocks. The veins cross-cut spinel harzburgite and consist mainly of opx with minor olivine, clinopyroxene (cpx) and/or amphibole. We identify 'rapidly crystallized' veins that cut wall-rock olivine without petrographic evidence of reaction, and 'reactive' veins subdivided into 'thick' (0*5-1 mm) and 'thin' (<€0*5 mm). Minerals in the rapidly crystallized veins are depleted in rare earth elements (REE) and high field strength elements (HFSE) and enriched in fluid-mobile elements relative to REE. Minerals in the reactive veins have higher Ti, Al, Cr and alkalis than minerals in the rapidly crystallized veins, as well as highly variable trace element abundances, especially in reaction zones, thin veins and related metasomatic pockets in the host peridotite. They commonly show U-shaped REE patterns and positive Zr and Hf spikes in normalized trace element patterns. Our data, supported by recent reports, show that the rapidly crystallized veins formed between 1200°C and 900°C from a liquid derived by fluid-fluxed melting of a refractory (harzburgitic) mantle source depleted in heavy REE. The reactive veins formed via 'fractionation-reactive percolation' from fractionated hydrous derivatives of the melts that precipitated the rapidly crystallized veins. These liquids re-equilibrated with the host through diffusion and fluid-assisted dissolution-precipitation reactions, whose end-products are thin reactive veins and metasomatic pockets with distinctive U-shaped REE patterns and Zr-Hf spikes. Some Avacha xenoliths contain veins of Fe-rich amphibole deposited from the host magma that penetrated fractures in the peridotite fragments during their transport to the surface. These products of contamination were mistakenly attributed to mantle metasomatism in previous studies of other suites of Avacha mantle xenoliths. Trace element abundances in such veins are higher than for reactive veins of mantle origin, but both have similar trace element patterns (U-shaped REE patterns and Zr-Hf spikes) suggesting that 'fractionation-reactive percolation' also took place during their formation and is common during interaction of refractory peridotites with percolating melts and fluids. Metasomatic pockets of cpx and amphibole replacing coarse opx and spinel in the host peridotites commonly occur in the vicinity of fractures that lead to reactive opx-rich veins. The cpx and amphiboles in the pockets show progressive depletion in middle REE and HFSE at constant light REE and/or large ion lithophile elements away from the veins towards the host. This indicates that residual hydrous fluids expelled from the source veins enriched the wall-rock peridotites in incompatible elements but were progressively modified by reaction with the host with increasing percolation distance. This process produces disseminated pockets of metasomatic minerals with a broad range of compositions from a single initial liquid and strongly affects the trace element budgets of the harzburgite xenoliths from Avacha. We show that melts and fluids are likely to undergo profound transformation as they travel through and react with the refractory host mantle, even on a millimetre to centimetre scale. The composition of the initial metasomatizing agents can only be inferred from the composition of the metasomatic phases in mantle rocks combined with appropriate partition coefficients if these phases come from well-equilibrated mineral assemblages located close to melt and/or fluid sources.

The transition from single layer to foliation boudinage: A dynamic modelling approach

Foliation boudinage is a deflection of foliation in the vicinity of a central discontinuity in foliated rocks, mostly filled with vein material. It shows evidence for brittle deformation and void-opening during ductile flow. We used a two-dimensional visco-elastic spring model based on a discrete element approach to study the dynamic development of foliation boudinage and the behaviour of anisotropic visco-elastic material deformed under pure shear conditions. The anisotropies are set by defining rheological heterogeneities in the models with (1) a single layer in a weaker matrix; (2) multi-layers with different elastic properties and (3) random-distributed “micas”, rows of horizontally aligned elements with the same elastic properties. The models show the nucleation and propagation of fractures during ductile deformation and flow perturbation and localization in the vicinity of fractures. Voids localize along extension fractures, at intersections of conjugate shear fractures and in small pull-apart structures along shear fractures, which look identical to boudin necks in isolated stiff layers. Results show that strain localization is strong in models with lower elastic constants and anisotropy whereas it is distributed in the models with higher elastic constants and anisotropy in which asymmetric and complex geometries develop. The models successfully show the development of and transitions between a single layer, multi-layer and foliation boudinage that are common structures in high strain zones.

A Study of Femoral Neck Fracture Repair Using a Recombinant Human Bone Morphogenetic Protein-2 Directional Release System

Significant difficulties are caused by the delayed union of femoral neck fractures. To address this issue, we designed a new device that applies recombinant human bone morphogenetic protein-2 (rhBMP-2) to promote fracture union. A cannulated screw with holes was used to deliver rhBMP-2 to the fracture site. Fibrin glue was used as an adhesive agent to hold rhBMP-2 in the vicinity of fracture. RhBMP-2 was protected with polylactide-glycolide acid microspheres. RhBMP-2 release was evaluated to determine the effect of the improved screws. When polylactide-glycolide acid microspheres were used, 3.65% of the rhBMP-2 was released in the first 2 h, 5.17% was released within 8 h, and 8.95% was released within 24 h. In the microsphere + fibrin glue group, 1.15% of the rhBMP-2 was released in the first 2 h, 1.75% was released within 8 h, and 6.68% was released within 24 h. Over 42 days, about 76.75% of the rhBMP-2 was released when using fibrin glue, which was lower than the amount released using microspheres alone (91.75%). In dog, a faster repair rate was observed on the side with the improved screw than on the side with traditional screw. The directional release system described here can improve the process of fracture healing and is a promising technique for repairing femoral neck fracture.

The Complex Hydrothermal History of Granitic Rocks: Multiple Feldspar Replacement Reactions under Subsolidus Conditions

Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid^rock interactions that affect large volumes of the Earth’s continental crust.This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replacement reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts. This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (� An9) along polysynthetic twinning and intragranular fractures. The features of this replacement are characteristic of a dissolution^reprecipitation mechanism. Finegrained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid^ feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered.

Origin of pore-water isotopic anomalies near fractures in the tournemire shales

The present study addresses the influence of fractures on the stable isotope contents of pore-water in the argillaceous formation of Tournemire. An isotopic profile crossing a fracture was acquired by means of the vacuum distillation technique (at 50 °C and 150 °C) and by the vapour exchange method. In addition, mineralogical and petrophysical characterisations were carried out along the profile. Data obtained from vapour exchange experiments display a slight scattering, not linked to the presence of the fracture. The same trend is observed from data obtained from distillation at 150 °C. Conversely, the profile derived from distillation at 50 °C displays two distinct zones: when the samples were collected at less than 1 m from the fracture, the oxygen-18 contents of pore-water are close to those obtained by the two others techniques, while for samples located at more than 1 m, a clear depletion in heavy isotope can be observed. To understand the origin of this artefact, the variation of several mineralogical and petrophysical parameters was studied. Neither the mineralogy of the bulk rock nor the composition of the clay fraction display any change as a function of the distance to the fracture. In fact, an increase of water content in the vicinity of fracture, which is associated with an increase of the proportion of large pores in samples, could account for the isotopic anomalies observed in data derived from vacuum distillation at 50 °C. This higher proportion of large pores at the vicinity of the fracture should rather make the water extraction easier, limiting the effects of incomplete distillation. This should lead to water isotope contents close to those derived from both vacuum distillation at high temperature and vapour exchange method.

Garnets as open systems during regional metamorphism

If a garnet fractures during metamorphism, its interior will be in chemical communication with matrix phases (solids and fluids) and may participate in late prograde or retrograde metamorphic reactions. The core of a growth-zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally, in the vicinity of fractures, or extensively, resulting in dissolution and reprecipitation of the garnet interior and/or production of new phases. The results of petrologic techniques such as thermobarometry, pressure-temperature path calculations, and geochronometry may be misinterpreted if sites of garnet and mineral-inclusion compositional modification are not identified. Brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions. The conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure, retrograde pressure-temperature path, and thermoelastic properties of the inclusion and garnet. Inclusion-induced fracturing is favored by nearly isothermal decompression, a common metamorphic pressure-temperature path. Other fractures may form as a result of propagation of an inclusion-host boundary or by mechanisms external to the garnet.

Flow path studies in forested watersheds of headwater tributaries of Brush Brook, Vermont

An investigation was undertaken into how headwater tributaries of Brush Brook, Vermont, could have average pH differences of almost two units (4.75 and 6.7). Sampling along four tributaries revealed that most of one tributary, below an area of seeps, had consistently higher pH, Ca2+, Mg2+, and K+, and lower Al than other sites. Bedrock mapping showed numerous fractures in vicinity of the seeps. A portion of this tributary's watershed and a portion of an acid tributary's watershed were intensively mapped for soil depth. Sampling showed the widespread existence of dense basal till in the watershed of the acid tributary but none in that of the near‐neutral stream. Lateral flow, found above the dense till, was chemically similar to that of the acid tributary and to solutions sampled from soil B horizons. There were no differences in the average pH of nonseep soils sampled from either watershed. Flow paths are hypothesized to be through the B horizons in the acid tributaries and from below the soil profile in the near‐neutral tributary. The acid catchment should be more sensitive to environmental change.